1. Field of the Invention
The present invention relates to a three-phase motor for a domestic appliance, wherein the motor is fed through an inverter. More particularly, the three-phase motor is utilized for the direct drive of washing machines and other domestic appliances.
In the provisions of drives in domestic or household appliances, the electrical three-phase motor is of increasing interest, in which the rotary field is produced from the household power supply through an intermediate direct current circuit through the intermediary of a high-frequency controlled inverter, which supplies the field windings of the stator with opposite phase shiftings.
In order to be able to attain the smoothest possible running in a household appliance; for instance, when in the case of a centrifugal drier there can be encountered high rotational speeds or considerable imbalances, the motor is regularly built into the housing of the appliance on rubber footings or the like, and through these is thereby electrically insulated with regard to this housing. The motor drives the operative installation of the appliance, such as the drum of a washing machine or drier. For this purpose, there can be provided a belt transmission extending from the rotor of the motor to the drive pulley of this operative installation, so that also in that regard is the motor insulated from the appliance housing.
In the future, a direct drive will increase in significance, in which the rotor is in the form of a pot-shaped or funnel-shaped external rotor, which at its exposed end surface is directly connected with the operative installation, generally such as the washer drum, or is even constructed integrally therewith. Inasmuch as the rotating operative installation is supported within the housing, in such a direct drive there is provided an ohmic (electrical) connection from the rotor across the support with the appliance housing.
Independently thereof, as to whether this connection is provided, or if not (such as with the usual internal rotors with belt drive), the housing due to appropriate safety regulations must be permanently connected to ground potential through a protective conductor.
The field coils of an inverter-controlled three-phase motor do not only carry the impressed rotary field currents, but moreover in comparison to their periodicity, extremely high-frequency currents due to the high-frequent actuation of the bridge circuits in the inverter. These high-frequency currents flow as capacitive shunt or leakage currents across the parasitic capacitances, which are constantly produced between electrically mutually insulated but sufficiently closely neighboring electrically-conductive constructional components; in effect, initially from the insulated field coils onto the exciter sheet packet which is built into the motor housing, and from there then further to the rotor which is spaced therefrom. When this relates to the construction of the above-mentioned external rotor, the high-frequency ohmic currents flow from the rotor across the support thereof to the appliance housing, and then further through the protective conductor which is connected to ground, and the household power supply back into the inverter. This flow of current through the support to the grounded protective conductor adversely influences the function and operating life of the support, and consequently in the interest of obtaining an undisrupted long term operation of the appliance, is extremely undesirable. When the protective conductor is not low-ohmically connected to the appliance housing, or due to a defect is even interrupted, then the housing acts as a large-surfaced radiating antenna for a high-frequency interference radiation; which is not permissible, in any instance, inasmuch as such a massive disruption can no longer be controlled through protective measures available in the circuitry technology, such as a power supply or mains filter. Consequently, when a person touches this housing, then his body due to the insufficient installation of the protective conduit, produces an auxiliary connection to ground potential, so that the body of the person is subjected to an extremely uncomfortable, and eventually even a health-endangering high-frequency current flow.
However, even in the case when the rotor is electrically insulated with regard to the appliance housing, such as in the instance of the internal rotor with belt drive, this can still lead to not negligible high-frequency capacitive shunt or bypass current across the protective conductor; namely from to field windings across the sheetmetal packet in the stator and through the leakage capacitances between the stator or, respectively, the motor housing to the surrounding constructional components of the grounded appliance. This shunt or bypass current is; in effect, not as high as that passing through the bearing construction of an external rotor; however, due to the high ohmic capacitive path from the motor housing to the appliance housing with its protective conductor, the stator sheetmetal packet of the motor now impermissibly radiates interference or leakage radiation as a particularly highly effective antenna.
2. Discussion of the Prior Art
In German Publication DE 34 39 894 A1 (Siemens) there is ascertained the problem that through the leakage capacitances between the stator field coils and the stator sheetmetal packet in the motor housing, capacitative currents flow off to a protective conductor, which is connected to the motor housing. This high frequency interference current circuit closes itself across the control circuit for the inverter, and as a result, threatens the function of the control circuit. In order to reduce this danger, it is proposed therein that the control circuit between the motor housing and the inverter be capacitively bridged over, in effect, conductively for the high frequency, so that the high frequency shunt or bypass current, such as a leakage current, can no longer endanger the control circuit.
However, the foregoing will not prevent that the capacitive shunt current will at all enter the protective conductor, which must also be positively connected to the appliance housing, with the above-mentioned endangerment of persons in the case of an inadequate connection; and the problem that for an external rotor as in the case of the direct drive, the support location for the rotor can be impermissibly loaded or stressed by the shunt or bypass current, is not even recognized in this publication.
In the disclosure of Miyazaki et al. U.S. Pat. No. 6,151,228, with the aid of an inductive transductor in the intermediate current circuit upstream of the inverter, there is effected a quantitative detection of the capacitive shunt or bypass current in the protective conductor, which is connected to a housing in which the motor operates. The transducer controls an amplifier circuit which supplies an oppositely poled current of the same time interval into the protective conductor, and thereby should compensate for the shunt current, so as to avoid an undue influence over the control circuit for the inverter. From the standpoint of circuit technology, this is extremely complex and critical in its manner of functioning through an intended broader working range for the amplifier circuit.
In accordance with the present invention there is prevented a straining or stressing of the bearing or support for a direct drive-external rotor with an ohmic leakage current to the appliance housing, as well as for an insulatedly-driven internal rotor-equipped motor, in that there is an absence of subjecting the appliance housing to the mentioned leakage current by means of the stray capacitances and in any event every disruptive current flow in the protective conductor is hereby precluded, in that already within the motor there is effected any leakage through the intermediary of circuitry; namely, directly from the stator back to the control of the inverter ahead of the stator field windings.